Pressure-induced phonon freezing in the Zn_1−x Be_xSe alloy: A study via the percolation model

We use the 1-bond → 2-phonon percolation doublet of zinc-blende alloys as a “mesoscope” for an unusual insight into their phonon mode behavior under pressure. We focus on (Zn,Be)Se and show by Raman scattering that the application of pressure disables an anharmonic coupling between the nominal Zn-Se zone-center TO mode and disorder-activated two-phonon zone-edge continua. This reveals an unexpectedly clear percolation-type fine structure of the Zn-Se TO mode, the latter fine structure being unaffected by pressure. In contrast, the original Be-Se doublet at ambient pressure, of the stretching-bending type, turns into a pure-bending singlet at the approach of the high-pressure ZnSe-like rocksalt phase, an unnatural one for the Be-Se bonds. The “freezing” of the Be-Se stretching mode is discussed within the scope of the percolation model (mesoscopic scale), supported by ab initio in sight into the pressure dependence of vibrational properties of the ultimate percolation-type Be-impurity motif (microscopic scale). Similar ab initio calculations are performed with (Ga,As)P, for reference purpose